1. Field of the Invention
This invention relates generally to rotary power transmission systems, and more particularly to transmissions utilizing a fluid coupling and a lockup clutch to lock the impeller and output turbine of the fluid coupling together under predetermined operating conditions. Even more particularly, the present invention relates to the use of such a fluid coupling and lockup clutch in combination with a hydromechanical transmission usable for high torque applications such as those encountered in transmitting power from the engine to the driving wheels of a tractor.
2. Description of the Prior Art
Hydromechanical transmissions are well known and have been used for some time in tractor applications, with a typical hydromechanical transmission being described in U.S. Pat. No. 4,138,907, which is assigned to the same assignee as the present invention. In addition, the use of lockup clutches is also known, particularly in conjunction with torque converters of automobile transmissions and the like, as described in U.S. Pat. Nos. 3,693,478; 3,809,956; 4,033,202; 4,056,177; and 4,152,984. However, no attempt has been made to utilize a lockup clutch in a fluid coupling, particularly in a fluid coupling utilized in a hydromechanical transmission such as a transmission to apply power from an engine to the driving wheels of a tractor.
In a typical tractor transmission, such as, for example, the one described in the aforesaid U.S. Pat. No. 4,138,907, the engine is coupled to a hydromechanical transmission which is, in turn, coupled to a variable ratio mechanical transmission via a master clutch. The output of the variable ratio mechanical transmission drives the driving wheels of the tractor, and the hydromechanical transmission is generally coupled directly to the flywheel of the engine, but in some instances, a fluid coupling may be used to couple the engine to the hydromechanical transmission.
While such systems have been used successfully in tractors and other vehicles, tractors employing a transmission that is directly coupled to the engine are difficult to control at low speed, and require that the master clutch be slipped when the tractor is moved slowly. Such clutch slipping results in wear and deterioration of the master clutch, and also in a bucking of the tractor.
In an attempt to improved the controllability of tractors and the like at low speed, some manufacturers have utilized a fluid coupling between the engine and transmission, because a fluid coupling has a certain amount of slip which reduces the tendency of a tractor to buck and improves low speed control. However, such slip is detrimental in that it reduces the fuel efficiency of the tractor, and thus, the design of the fluid coupling requires a trade-off between low speed operation and fuel efficiency, with greater slip favoring low speed operation at the expense of fuel efficiency.